Journal of Membrane Science 323 (2008) 207–212

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

Improved Ti-silicate umbite membranes for the separation of H2

lezArago

e (K2

s menes wher meatiohe sa4.6 ±

Víctor Sebastiána, Zhi Linb, João Rochab, Carlos Téla Department of Chemical and Environmental Engineering and Nanoscience Institute ofb Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

a r t i c l e i n f o

Article history:Received 16 May 2008Received in revised form 10 June 2008Accepted 11 June 2008Available online 20 June 2008

Keywords:UmbiteTitanosilicateZeoliteZeolite membraneHydrogen separation

a b s t r a c t

Microporous titanosilicatas 5-�m-thick continuousynthesis. These membraence of moisture, unlike otstrongly hinders gas permmembranes obtained in taverage H2 permeance of

1. Introduction

Hydrogen purification and separation has been identified as abottleneck in the development of advanced hydrogen technologies.Consequently, many techniques for hydrogen separation are in use

or under study, such as pressure swing adsorption [1], cryogenicseparation [2], catalytic purification [3], and selective diffusionthrough membrane interfaces [4]. Palladium is still the membranematerial of choice in many applications as a result of its highH2 selectivity, good mechanical characteristics and high catalyticsurface [5,6]. Hydrogen transport through the membrane can bedescribed by the well-known solution-diffusion mechanism: themolecule is adsorbed on one side of the membrane, splits into twoatoms, diffuses through the metal bulk, and recombines and des-orbs at the permeate side. Unfortunately, palladium and its alloysare extremely expensive, roughly twice the cost of gold, makingthem impractical for large-scale applications [7]. To improve fluxesand reduce membrane cost, thin Pd layers deposited on porousceramic or metallic supports are used [6]. However, because of theircharacteristics, these thin Pd films are intrinsically prone to thedevelopment of defects such as pinholes or cracks, with a strongimpact on membrane selectivity. This problem is exacerbated bythe development of brittleness in the Pd film due to formation ofhydrides in the presence of H2. In addition, another major tech-nical disadvantage of palladium membranes in most applications

∗ Corresponding author. Tel.: +34 976 76 2471; fax: +34 976 76 1879.E-mail address: coronas@unizar.es (J. Coronas).

0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2008.06.033

a, Jesús Santamaríaa, Joaquín Coronasa,∗

n, University of Zaragoza, 50018 Zaragoza, Spain

TiSi3O9•H2O) with an umbite structure has been reproducibly prepared

mbranes on asymmetric TiO2 tubular supports by seeded hydrothermalere able to separate H2 from N2 with good selectivity even in the pres-icroporous zeolitic hydrophilic membranes where capillary condensationn. The average H2/N2 separation selectivity calculated for five Ti-umbiteme synthesis conditions (200 ◦C, 48 h) was 45.2 ± 3.8 (together with an1.1 × 10−9 mol/(m2 s Pa).

© 2008 Elsevier B.V. All rights reserved.

is their high sensitivity to chemicals such as sulphur, chlorine andeven CO.

As an alternative to Pd membranes, selective porous ceramicmembranes can be used. These generally have a two-layer struc-ture: the separation membrane itself and a thicker support layerwith wider pores. Microporous materials such as zeolites and ingeneral materials with subnanometric channels in their crystallinestructure can act as molecular sieves [8,9] with preferential per-

meation of small molecules. Hydrogen fluxes through this kindof membranes are promising, although many issues need to beresolved before this technology can be applied, including repro-ducibility in scale-up [10,11]. Another interesting alternative for H2gas separation through membranes has to do with the use of the so-called mixed matrix membranes where appropriate microporousinorganic fillers can enhance the performance of a H2-selectivemembrane polymer [12,13].

Several umbite-type synthetic materials have been reported inthe last few years: K2ZrSi3O9

•H2O [14,15], K2TiSi3O9•H2O [16–18],

K2SnSi3O9•H2O [19] and K2ZrGe3O9

•H2O [20]. Like titanosilicatesETS-10 and ETS-4 [21], umbite-type compounds possess frame-work structures built of MO6 (M = Zr, Sn, Ti) octahedra and SiO4(or GeO4) tetrahedra. These materials can be reversibly dehydratedand are good cation exchangers. In addition, owing to the fact thatthe one-dimensional eight-membered ring channel is occupied byK+ cations and water molecules, Ti-umbite does not adsorb N2.

The small pore size of umbite-type materials grants them poten-tial for separation of H2 containing mixtures. After some previousworks reporting on the synthesis, characterization and H2 separa-tion properties of the first titanosilicate umbite membrane [22,23],

embra

T

208 V. Sebastián et al. / Journal of M

here we present in detail the preparation of an improved secondgeneration of Ti-umbite membranes. Particularly, the new seededliquid phase hydrothermal synthesis method used here representsa significant advance, as it employs Cl-free nutrient sources, thusavoiding the problems related to KCl co-crystallization on the mem-brane.

2. Experimental

Ti-umbite membranes were prepared on tubular porous TiO2asymmetric supports (Inocermic GmbH) with 5 nm pore diameter.In this case the separation layer was on the inner surface of thetubes, �-alumina being the macroporous part of the support. Theinternal and external diameters of the tubes were 7 and 10 mm,respectively. The tubes were 8 cm long (5 cm permeable).

The preparation procedure is shown in Fig. 1. The membraneswere prepared using the following gel molar composition: 2.2K2O:4.5 SiO2:TiO2:180 H2O. The precursor reactants were titanium(IV) isopropoxide (Ti[OCH(CH3)2]4; 97 wt.%, Aldrich), KOH (85 wt.%,Merck), fumed silica (Aerosil 300, Degussa) and deionized water.To prepare 40.85 g gel, Aerosil 300 (2.88 g) was added to a solutionof KOH (2.88 g) and deionized water (8.90 g). After 10 min of stir-ring a clear solution was obtained. Then, Titanium (IV) isopropoxide(2.89 g) was added under vigorous stirring. A white viscous solu-tion was obtained, and after 20 min, deionized water was added

(23.30 g) stirring for 30 min.

The membranes were prepared by subjecting the seeded sub-strate to a secondary growth step. For inner-surface seeding, theouter surface of the tubes was wrapped with Teflon tape (thiswrapping will be kept up to the end of the hydrothermal synthe-sis procedure). The TiO2 asymmetric tubes were immersed once inan ultrasound bath for 15 min with a water suspension containing5 wt.% umbite powder which was previously prepared with an aver-age particle size of circa 300 nm, as measured by photon correlationspectroscopy, PCS, Malvern Zetasizer 3000 HS [23]. After drying thetubes at 120 ◦C, an optimum seeded layer was achieved. The seededsupport was placed vertically in a Teflon-lined autoclave where thesynthesis mixture (35 mL) was poured. The hydrothermal synthesiswas carried out at 200 ◦C for 48 h keeping the autoclave still. Thesynthesis was terminated by quenching the autoclave in runningwater. The membrane was washed with water at room temperatureand dried overnight at 100 ◦C.

The Ti-umbite membranes were characterized by scanningelectron microscopy (SEM, JEOL 6400 operating at 20 kV) and X-ray diffraction (XRD, Philips X’Pert MPD diffractometer using CuK�1 radiation, � = 1.5418 Å, and graphite monochromator). Mem-

Fig. 1. Procedure to obtain a microporous titanosilicate Ti-umbite membrane.

ne Science 323 (2008) 207–212

branes, previously pretreated in Ar (99.999% purity) flow at 170 ◦Cfor 12–14 h, were also tested for the separation of 50/50 H2/N2(both having 99.999% purity) gas mixtures. The membrane to betested was placed in a stainless steel module and sealed withsilicone o-rings. The module was heated in an electrical oven. A70/70 cm3(STP)/min H2/N2 mass-flow controlled (Bronkhorst Hi-ec, F-201C-FA-22-V) stream was fed into the membrane side

(where the Ti-umbite layer was synthesized). The permeate sidewas swept with 135 cm3(STP)/min of Ar to create the necessarydriving force. The total pressure drop across the membrane wasusually zero. The exit streams from the retentate and permeatesides were analyzed by an on-line mass spectrometer (OmniStar,Pfeiffer Vacuum QMS 200).

The performance of some membranes was also tested in pres-ence of H2O/H2/N2, IPA/H2/N2 and NH3/IPA/H2/N2 mixtures, IPAbeing isopropanol, after passing the H2/N2 gas stream through abubble saturator at 40 ◦C filled with water, isopropanol (Aldrich)or 2 M NH3 solution in isopropanol (Aldrich). At 40 ◦C (the cur-rent temperature for the permeation experiments) the waterand isopropanol relative pressures achieved were around 0.7.For comparison, a high-quality mordenite membrane (with aH2O/EtOH pervaporation separation factor of 1360 and a waterflow of 0.06 kg/(m2 h)) was evaluated under the same conditionsfor H2/N2 separation in presence of moisture. This mordenitemembrane was prepared following the instructions reported in[11].

The separation selectivities quoted below were calculated aspermeance ratios, using the log-mean partial pressure differenceacross the membrane to obtain the permeances. Mass balance clo-sures for the different species based on the composition and flowrate of the feed and the two exit streams were better than 5%.

3. Results and discussion

3.1. Improved Ti-umbite membranes

Table 1 shows some relevant data of Ti-umbite membranes.According to the Ti-umbite weight gain measured after thehydrothermal synthesis procedure, it can be inferred that the crys-tallization process is highly reproducible giving rise to an averageweight gain of 16.6 ± 0.3 mg of Ti-umbite per g of support. One of themost interesting facts is the achievement of a good and again highlyreproducible H2/N2 selectivity with an average value of 45.2 anda standard deviation of 3.8. These results are considerably betterthan those published in our previous report [22], where the averageselectivity was 38.5 ± 7.5 (for four Ti-umbite membranes preparedalso on TiO2 supports). The improvement can be explained as aresult of the halide-free synthesis used here, which allowed toobtain a pure Ti-umbite membrane, without the obstacle relatedto the KCl co-crystallization [22].

The H2 permeance is also quite reproducible, the averagevalue being 4.6 ± 1.1 × 10−9 mol/(m2 s Pa). On the other hand, thisvalue is not as high as that measured in our previous work

Table 1Some relevant data of Ti-umbite membranes

Membrane H2 permeance ×109

(mol/m2 s Pa)H2/N2 selectivity Weight gain

(mgumbite/gsupport)

TiM1 4.9 ± 0.4 47.2 ± 1.2 16.7TiM2 3.8 ± 0.6 39.7 ± 0.8 16.8TiM3 6.5 ± 1.2 47.1 ± 0.5 16.2TiM4 3.6 ± 0.5 43.0 ± 1.2 16.4TiM5 4.4 ± 0.7 49.1 ± 0.6 16.8Average 4.6 ± 1.1 45.2 ± 3.8 16.6 ± 0.3

The permeances were obtained at 40 ◦C.

V. Sebastián et al. / Journal of Membra

Fig. 2. XRD patterns of Ti-umbite powder and Ti-umbite membrane where theasterisk points out the �-alumina support reflection.

(6.7 ± 5.3 × 10−8 mol/(m2 s Pa)). Considering that there is no varia-tion in the thickness (5 �m coinciding with that already published[22]) and crystallinity (see Fig. 2 to assess the Ti-umbite layer crys-tallinity by comparison with the corresponding powder) of the

Fig. 3. SEM images of Ti-umbite membranes: (a and b) top views of a Ti-um

Fig. 4. SEM images of Ti-umbite membranes prepared using TiCl3 as Ti source: (a) top vTi-umbite membrane.

ne Science 323 (2008) 207–212 209

Ti-umbite membrane, the permeance decrease must be attributedto a lower density of defects, which would also be consistentwith a higher selectivity. As we already reported for Ti-umbite

membranes [22], in Fig. 2, the intensity of the (2 0 0) peak at17.8◦ 2� significantly increases when compared to the powder,suggesting an a-out-of-plane preferential orientation, while theintensities of the (0 2 0) and (0 0 2) reflections at 13.6◦ and 25.0◦

2�, respectively, noticeably decrease, indicating that b and c axesare in-plane, in accordance with an a-out-of-plane preferential ori-entation.

As shown in Fig. 3, a smooth surface with a high intergrowth ofTi-umbite tiny crystals was obtained (Fig. 3a and b), and thereforeit should help to produce a more continuous membrane withoutinter-crystalline defects. The corresponding cross-section (Fig. 3cand d) shows a continuous, defect-free Ti-umbite membrane, withthe aforementioned thickness of 5 �m. However, although the goodcrystal intergrowth observed in these images is a positive charac-teristic of our Ti-umbite membranes, the increasing a-out-of-planepreferential orientation may be detrimental to the permeance ofthe membranes and affect their gas separation properties, sincethe one-dimension eight-membered ring channel (2.7 × 5.5 Å) inumbite materials is perpendicular to direction a [18]. For compari-son, Fig. 4a shows a SEM image of a Ti-umbite membrane preparedusing TiCl3 as Ti source [22], where all the cubic crystals observed

bite membrane and (c and d) cross-sections of the same membrane.

iew of an as-synthesized Ti-umbite membrane and (b) top view of gently washed

embra

Temperature = 40 ◦C.

in inter-crystalline defects. Due to this, a decrease of about 15% canbe observed in the H2 permeance (which gives an estimation ofthe contribution of inter-crystalline spaces to H2 flux), and the N2permeance suddenly decreases. As a consequence, the H2/N2 selec-tivity increased from 48.5 to 800 after 55 h and to 877 after 120 h. Itshould be also taken into consideration that under moisture condi-tions the membrane would be re-hydrated to some extent. This wasverified in a longer experiment (644 h) discussed in our precedentreport [22].

As shown in Fig. 6a for the same membrane sample of Table 2(TiM5), at 140 ◦C the levels of H2 permeance and H2/N2 selec-tivity of the dry membrane are comparable to those values andtrends observed for Fig. 5 membrane TiM1. Upon heating underthe H2O/H2/N2 ternary mixture, the H2/N2 selectivity (see Fig. 6b)decreased to a value comparable to that of the dry membraneaccounting for an important diminution in the water relative pres-

210 V. Sebastián et al. / Journal of M

correspond to KCl. Even if after gently washing some KCl remained(Fig. 4b) the dissolution of this salt present between umbite crystalsmight produce inter-crystalline defects.

3.2. Separation of H2/N2 mixtures

Fig. 5 shows the H2 permeance and the H2/N2 separation selec-tivity as a function of temperature for Ti-umbite membrane TiM1.The permeance of H2 is higher than that of N2 in the 30–150 ◦Ctemperature range. As expected considering the microporous char-acter of these membranes, both permeances are activated, i.e.they increase with increasing temperature. The H2/N2 selectivitydecreases with temperature (from 47.2 at 40 ◦C to 31.4 at 150 ◦C)because the N2 permeance increases faster than that of H2. Thisdifferent temperature activation may be explained assuming thatH2 (0.289 nm of kinetic diameter) and N2 (0.364 nm of kinetic diam-eter) permeate through different channels: N2 does not enter theumbite channel (which has a limiting dimension of 2.7 Å) and per-meates only through inter-crystalline voids. Given the selectivityvalues observed, and also the activated transport behaviour, thesedefects are expected to be of a size similar to that of the N2 molecule.As temperature increases from 40 to 150 ◦C, mechanical tensionsdue to thermal expansion cause a slight enlargement of the inter-crystalline voids, leading to the increase in N2 permeance anddecrease in H2/N2 selectivity observed.

3.3. Separation of vapour/H2/N2 mixtures

As shown in our previous study [22], the water content inTi-umbite membranes would tend towards stabilization. In fact, Ti-umbite is a hydrophilic material with a theoretical expected loss ofwater of 4.8 wt.% based on its K2TiSi3O9

•H2O formula. As describedby Zou and Dadachov [18], besides the eight-ring channels consist-ing of alternating SiO4 tetrahedra and TiO6 octahedra, there aresix-ring channels formed by alternating SiO4 tetrahedra pairs andTiO6 octahedra. Each eight- and six-ring channel has a pair of K+

cations, while there are two molecules of water in the eight-ringchannels only. It seems evident that the loss of water should affectthe umbite pore size; in fact, some loosely bonded water can be des-orbed below 185 ◦C (under thermogravimetric analysis conditions)[24], returning to the idea exposed above on the water content sta-bilization in long term experiments. Table 2 shows the behaviourof a Ti-umbite membrane (TiM5) with time on stream under dif-ferent feed conditions. For the first 50 h, the feed contains a dryH2/N2 mixture, and a stable behaviour is observed. The feed is thenswitched to a H2O/H2/N2 mixture in which the water relative pres-sure was 0.7, high enough to ensure capillary condensation of water

Fig. 5. H2/N2 separation selectivity and H2 and N2 permeances through membraneTiM1 as a function of temperature. Lines are guide to the eye only.

ne Science 323 (2008) 207–212

Table 2Separation of mixtures containing H2 through membrane TiM5

Time on stream (h) Gas mixture H2 permeance ×109

(mol/m2 s Pa)H2/N2 selectivity

16 H2/N2 4.4 ± 0.7 49.1 ± 0.630 H2/N2 4.5 ± 0.6 48.5 ± 0.650 H2O/H2/N2 3.9 ± 0.6 632 ± 455 H2O/H2/N2 3.8 ± 0.5 800 ± 6

120 H2O/H2/N2 3.7 ± 0.4 877 ± 8

sure (the value of 0.7 relative pressure corresponds to 40 ◦C, whileat 140 ◦ it would be switched to approximately 0.01) and hence for

Fig. 6. Separation of mixtures containing H2 through membrane TiM5. Tempera-ture = 40 ◦C: (a) H2 permeance and (b) H2/N2 separation selectivity. IPA refers hereto isopropanol. Lines are guide to the eye only.

V. Sebastián et al. / Journal of Membra

Fig. 7. Ti-umbite (TiM5) versus mordenite membrane as a function of temperature:(a) H2 permeance and (b) H2/N2 separation selectivity. Lines are guide to the eyeonly.

the disappearance of the capillary condensation effect on the N2transport through the membrane.

Two further experiments were made by bubbling the H2/N2gas mixture through pure isopropanol (IPA) and a NH3/IPA solu-tion (see Fig. 6). The pure IPA had no influence in the H2/N2separation behaviour, partially because its kinetic diameter (4.8 Å)is considerably larger than the pore size of the Ti-umbite mem-brane (taking into consideration both inter- and intra-crystalline

pores). As a result, the data on permeance and selectivity obtainedwith IPA/H2/N2 mixtures practically reproduce those obtained withH2/N2 feeds. On the other hand, NH3, even having a kinetic diam-eter of 2.6 Å, had a smaller influence than water on the H2/N2selectivity, and at low temperature only a moderate increase inthe H2/N2 selectivity was observed, reaching a value of 85.3. Thiscan be explained if one takes into account that NH3 is only weaklyadsorbed on the Ti-umbite structure compared to water [25], andwe must conclude that the hydration of the Ti-umbite structure isthe key factor to understand the important increase of the H2/N2selectivity in presence of moisture. In fact, the hydrated K+ radiusis 3.8 Å (versus 1.33 Å for the dry K+ radius). In consequence, even ifwater capillary condensation was not produced in Ti-umbite pores,a change in the hydration sphere of its cations might exert animportant effect on the transport behaviour of the membrane. Onthe contrary, NH3 would not produce solvation effects on the K+

cation.Note that, unlike other microporous membranes [26,27], water

should not permeate through the intrinsic micropores of thezeolitic material (Ti-umbite) but undergoes capillary condensationin the defects existing in the membrane. The size and number

ne Science 323 (2008) 207–212 211

of these defects should be very limited, since the selectivity ofthe membrane without water is high. Finally, a mordenite (MOR-type zeolite) membrane was tested using the same H2O/H2/N2ternary mixture as to the Ti-umbite membrane. MOR-type zeo-lite is hydrophilic and as such is often used to separate waterfrom its mixtures with organic compounds such as ethanol [11].Mordenite, unlike Ti-umbite has micropores large enough to allowthe passage of water. Given the hydrophilic character of MOR-typezeolite membranes, water adsorbs strongly, and can block the per-meation of other molecules. As shown in Fig. 7a, at low temperaturewater hindered the H2 transport through the mordenite membrane(the concentration of H2 in the permeate side was close to thethreshold of detection for the mass spectrometer); H2 transportwas progressively restored as the temperature increased weak-ening the adsorption of water on mordenite. Also, the mordenitemembrane was not able to produce a H2/N2 separation selectivityhigher than 7 (Fig. 7b), in contrast with the behaviour of the Ti-umbite membrane which not only kept its H2 permeance level butalso augmented its H2/N2 separation selectivity in the presence ofmoisture.

4. Conclusion

Microporous titanosilicate (K2TiSi3O9•H2O) with an umbite

structure can be prepared as continuous membranes on porousasymmetric TiO2 tubular supports by seeded hydrothermal synthe-sis. The low density of defects achieved with this type of synthesisconditions makes it possible to obtain good quality membranes forthe separation of H2/N2 mixtures. Unlike other zeolitic membranes,moisture did not seem to permeate through the intrinsic microp-ores of Ti-umbite but used the inter-crystalline defects existing inthe membrane where it was capillarily condensed. This gave riseto a tremendous increase of the H2/N2 selectivity (from close to50 to more than 800) in the presence of a sufficiently high partialpressure of water, while having only a relatively small impact onthe H2 permeance. Under the same conditions of presence of mois-ture in the feed a hydrophilic mordenite membrane was not able toperform adequately for H2 permeation: its H2/N2 separation selec-tivity being always below 7 and its level of H2 permeance practicallynegligible at the same temperature of 40 ◦C.

Acknowledgements

Financial support from Spanish Ministry of Science and Edu-cation (MAT2007-61028) is gratefully acknowledged. Z.L. and J.R.acknowledge the funding from FCT and PTDC of Portugal andFEDER.

References

[1] A.H. Malek, S. Farooq, Hydrogen purification from refinery fuel gas by pressureswing adsorption, AIChE J. 44 (1998) 1985–1992.

[2] C.P. Huang, A. T-Raissi, Analyses of one-step liquid hydrogen production frommethane and landfill gas, J. Power Sources 173 (2007) 950–958.

[3] O. de la Iglesia, V. Sebastian, R. Mallada, G. Nikolaidis, J. Coronas, G. Kolb, R.Zapf, V. Hessel, J. Santamaria, Preparation of Pt/ZSM-5 films on stainless steelmicroreactors, Catal. Today 125 (2007) 2–10.

[4] J.A. Ritter, A.D. Ebner, State-of-the-art adsorption and membrane separationprocesses for hydrogen production in the chemical and petrochemical indus-tries, Sep. Sci. Technol. 42 (2007) 1123–1193.

[5] F. Roa, J.D. Way, Influence of alloy composition and membrane fabrication on thepressure dependence of the hydrogen flux of palladium–copper membranes,Ind. Eng. Chem. Res. 42 (2003) 5827–5835.

[6] C.L. Su, T. Jin, K. Kuraoka, Y. Matsumura, T. Yazawa, Thin palladium filmsupported on SiO2-modified porous stainless steel for a high-hydrogen-fluxmembrane, Ind. Eng. Chem. Res. 44 (2005) 3053–3058.

[7] T.M. Adams, J. Mickalonis, Hydrogen permeability of multiphase V–Ti–Ni metal-lic membranes, Mater. Lett. 61 (2007) 817–820.

embra

[

[

[

[

[

[

[

212 V. Sebastián et al. / Journal of M

[8] G.Q. Guan, T. Tanaka, K. Kusakabe, K.I. Sotowa, S. Morooka, Characterization ofAIPO(4)-type molecular sieving membranes formed on a porous alpha-aluminatube, J. Membr. Sci. 214 (2003) 191–198.

[9] J. Caro, M. Noack, Zeolite membranes—recent developments and progress,Micropor. Mesopor. Mater., 2008 doi:10.1016/j.micromeso.2008.03.008.

10] L. Gora, N. Nishiyama, J.C. Jansen, F. Kapteijn, V. Teplyakov, T. Maschmeyer,Highly reproducible high-flux silicalite-1 membranes: optimization ofsilicalite-1 membrane preparation, Sep. Purif. Technol. 22 (2001) 223–229.

[11] A. Navajas, R. Mallada, C. Tellez, J. Coronas, M. Menendez, J. Santamaria, Study onthe reproducibility of mordenite tubular membranes used in the dehydrationof ethanol, J. Membr. Sci. 299 (2007) 166–173.

12] S. Choi, J. Coronas, E. Jordan, W. Oh, S. Nair, F. Onorato, D.F. Shantz, M. Tsapatsis,Layered silicates by swelling of AMH-3 and nanocomposite membranes, Angew.Chem. Int. Ed. 47 (2008) 552–555.

13] P. Gorgojo, S. Uriel, C. Téllez, J. Coronas, Development of mixed matrix mem-branes based on zeolite Nu-6(2) for gas separation, Micropor. Mesopor. Mater.,2008 doi:10.1016/j.micromeso.2007.11.046.

14] D.M. Poojary, A.I. Bortun, L.N. Bortun, A. Clearfield, Synthesis and X-ray powderstructures of K2SnSi3O9

•H2O and its ion-exchanged phases with Na and Cs,Inorg. Chem. 36 (1997) 3072–3079.

15] S.R. Jale, A. Ojo, F.R. Fitch, Synthesis of microporous zirconosilicates containingZrO6 octahedra and SiO4 tetrahedra, Chem. Commun. (1999) 411–412.

16] B. Mihailova, V. Valtchev, S. Mintova, L. Konstatinov, Characterization of waterin microporous titanium silicates, J. Mater. Sci. Lett. 16 (1997) 1303–1304.

17] Z. Lin, J. Rocha, P. Brandao, A. Ferreira, A.P. Esculcas, D.J. Pedrosa de Jesus, Syn-thesis and structural characterization of microporous umbite, penkvilksite andother titanosilicates, J. Phys. Chem. B 101 (1997) 7114–7120.

[

[

[

[

[

[

[

[

[

[

ne Science 323 (2008) 207–212

18] X. Zou, M.S. Dadachov, K2TiSi3O9•HsO, Acta Crystallogr. Sec. C 56 (2000)

738–739.19] Z. Lin, J. Rocha, A. Valente, Synthesis and characterization of a framework micro-

porous stannosilicate, Chem. Commun. (1999) 2489–2490.20] J. Plevert, R. Sanchez-Smith, T.M. Gentz, H. Li, T.L. Groy, O.M. Yaghi, M. O’Keeffe,

Synthesis and characterization of zirconogermanates, Inorg. Chem. 42 (2003)5954–5959.

21] J. Rocha, M.W. Anderson, Microporous titanosilicates and other novel mixedoctahedral-tetrahedral framework oxides, Eur. J. Inorg. Chem. 5 (2000)801–818.

22] V. Sebastian, Z. Lin, J. Rocha, C. Tellez, J. Santamaria, J. Coronas, Synthesis, charac-terization and separation properties of Sn–and Ti-silicate umbite membranes,Chem. Mater. 18 (2006) 2472–2479.

23] V. Sebastian, Z. Lin, J. Rocha, C. Tellez, J. Santamaria, J. Coronas, A new titanosil-icate umbite membrane for the separation of H2, Chem. Commun. (2005)3036–3037.

24] V. Sebastian, I. Diaz, C. Tellez, J. Coronas, J. Santamaria, Spheres of microp-orous titanosilicate umbite with hierarchical pore systems, Adv. Funct. Mater.18 (2008) 1314–1320.

25] N. Navascues, E.D. Skouras, V. Nikolakis, V.N. Burganos, C. Tellez, J. Coronas,Reconstruction of umbite framework variants by atomistic simulations usingXRD and sorption data, Chem. Eng. Proc. 47 (2008) 1139–1149.

26] O.K. Aoki, K. Kusakabe, S. Morooka, Separation of gases with an A-type zeolitemembrane, Ind. Eng. Chem. Res. 39 (2000) 2245–2251.

27] J.C. Poshusta, R.D. Noble, J.L. Falconer, Characterization of SAPO-34 membranesby water adsorption, J. Membr. Sci. 186 (2001) 25–40.